Center pivot irrigators have been used in Australia since the 1960's. A typical center pivot irrigator consists of a pipeline comprising a series of pipeline spans having spray nozzles at spaced locations along each span, the spans being supported by various types of trusses wherein each span is located at an elevated position between wheeled carriages. The whole pipeline rotates about a pivot point and the supporting carriages travel around concentric circles in order to irrigate a circular area. Water is usually introduced into the pipeline at the center of the circular area. A linear or lateral type irrigator has the same basic structure as the center pivot irrigator except it does not travel in a circle but the carriages travel in a line to irrigate a rectangular area.
The existing center pivot and lateral irrigators have a number of disadvantages. For example, the wheel assembly of each carriage usually comprises two spaced wheels which can be manually moved between irrigation and transport positions. When in the irrigation position, the wheels are aligned with the circumferential or linear track that they are to travel and when in the transport position, they are moved through approximately 90.degree. . Thus, a towing vehicle can be coupled to the end of the irrigator and it can be towed from one location to another. In the present systems, it is a time consuming procedure to move the wheels, usually involving jacking the carriages or individual wheels and manually shifting them into position prior to transport and following the reverse procedure once the irrigator has been moved to the new location. Also the known irrigators do not tolerate large departures from straight line motion when being moved.
Generally, irrigators of the center pivot type only irrigate circular areas, however, one known design employs a corner watering device employing a trailing outer pipeline span which lags behind the main pipeline and is electronically controlled via an underground electric cable so that as the irrigator passes a corner, the lagging span travels out progressively into a radial position and then travels back into its lagging position as the irrigator passes the corner. This form of corner watering device is only partially successful. Another way of dealing with this problem is that in some irrigators, a large radially directed sprinkler is employed which is turned on as the irrigator passes a corner. This results in irrigation water being delivered outside the target area and/or the distribution of water in the corner being non-uniform. The first mentioned corner watering system involving the lagging span provides a more uniform coverage but is expensive and unreliable and consequently, is very uncommon.
A further problem inherent in conventional irrigators employing carriages is that the ground directly in front of the carriage wheels is watered and as the carriage wheels travel through this watered region "rutting", involving creation of circular grooves in the terrain, can occur which in turn can create later problems when moving the irrigator or during subsequent harvesting procedures.